django rest framework: set field-level error from serializer validate() method

Question

I have a serializer that validates fields based on the values of other fields, In the error response I would like to show each field error as a field error as opposed to showing everything under "non_field_errors" which is what would happen if I were to raise a ValidationError in the object-level validate method. Below is an illustration of what I'm trying to achieve:

MySerializer(ModelSerializer):     ...     def validate(self, data):         field_val1 = data['field_val1']         field_val2 = data['field_val2']         if not self._is_field_valid(field_val1, field_val2):             # The below line is how I would do what I want with Django             # Forms, however, it's not valid in DRF             self._errors['field_val1'] = 'this field is not valid' 

The desired error response is:

{'field_val1': ['this field is not valid']} 

Answer 1

I figured it out, on this page of the documentation in the "BaseSerializer" section, there's an example that shows ValidationError can take a dictionary argument upon initialization.

If I raise ValidationError({'field_val1': ['this field is not valid']}) I get the JSON response I want.

Answer 2

Similarly to the answer by @Jkk.jonah, this raises a ValidationError, but it reuses the original exception text without need to re-implement translations:

try:     serializer.fields['field_val1'].fail('required') except ValidationError as exc:     raise ValidationError({         'field_val1': exc.detail,     }) 

By default (i.e. on rest_framework.fields.Field class), available keys are:

default_error_messages = {     'required': _('This field is required.'),     'null': _('This field may not be null.') } 

Subclasses can add their own error messages there (and Serializer is a subclass of Field).

BTW, new error messages will be automagically merged with existing (inherited) messages - won't be overridden as might be expected.